The present invention relates to an apparatus and a method for restraining vibration of structures.
Formerly, in order to make structures resistant against such vibrational loads as earthquakes, winds and traffics, the structures have been constructed earthquake resistant by virtue of seismic design which is a design philosophy to construct the structure enough strong to withstand the loads. But recently, a notion so called a seismic isolation has been developed and brought into practical use for protecting the structure from seismic loads. The seismic isolation is to support the structure by a relatively soft and flexible support and absorb the seismic load to be induced to the structure by a large deformability of the support. A problem as to the seismic isolation is, though the isolation is effective for cutting off vibrational loads which is to be induced to the structure through the support, that it sometimes make the situation worse when a vibrational load is induced directly to the building without passing through the supports. Such situations occur when the structure vibrates because of winds or, for example, when a bridge vibrates because of the traffics passing thereon. In such cases, vibration is apt to become large if a conventional isolation technique is applied.
In order to give solution to this inconvenience, vibration restraining structures have also been developed. FIG. 9 shows an example of such conventional vibration restraining structures.
In FIG. 9, a vibration restraining structure denoted by 100 comprises a weight 101, a supporting bed 102 having an oil film 103 thereon for supporting the weight 101 by the film 103 and supported by a floor 104, a gas damper 105 containing nitrogen gas therein, a hydraulic actuator 106 for giving a horizontal force to the weight 101, a servo-valve 107 for controlling oil flow to the actuator 106, and an oil supply unit 108 for supplying oil to the actuator 106 through the servo-valve 107. The structure has a same sectional feature as regards two vertical planes perpendicular to each other.
Operation of the vibration restraining structure is as follows.
Because the weight 101 is supported by the supporting bed 102 and the oil film 103 thereon, friction force acting on the weight 101 is negligibly small. So, without a thrusting force of the actuator 106 and the gas damper 105, the weight 101 stays still even when the structure, thus the bed 102 vibrates horizontally. On the contrary, the actuator 106 receives a reaction force from the weight 101 when it exerts a thrusting force thereto. By giving thrust force in a proper direction, the reaction force restrains the vibration of the structure. This is how the vibration restraining structure 100 acts to restrain the vibration of the structure. The gas damper 105 increases the effect of the vibration restraining structure 100 by consuming vibrational energy.
A problem as to the above-mentioned vibration restraining structure is that a numbers of supplementary equipments, such as control units and sensors, are necessary for the restraining structure thus increasing inevitably the total cost of the restraining structure.
Another problem is that the number of the restraining structures installed in a structure is restricted to a very few numbers from an economical point of view. Therefore, the restraining force exerted to the structure by the restraining structure is concentrated on the few points where the restraining structure is installed and in some cases reinforcement of the structure is need.
One other conventional device for restraining vibration of a structure is invented by the present inventor and disclosed in a Japanese Patent Application No. 62-13367, as shown in FIG. 14. The device 110 comprises an enclosure 111, a weight 112, a spring 113 supporting the weight 112 from the enclosure 111, and viscous liquid 114 filling a space in the enclosure 111. The device is attached to a structure 115, vibration of which is to be restrained. When the structure 115 vibrates, the vibration stimulates the device 110 and the weight 112 starts oscillating at its natural frequency under effects of the viscous liquid 114 and the spring 113. If the natural frequency of the device 110 is so tuned as to coincide with a natural frequency of the structure, the device acts as a so called dynamic damper and reduces vibration of the structure. Natural frequency of the device 110 can be varied and adjusted to coincide with that of the structure by properly selecting properties of the weight 112, the spring 113 and the viscous liquid 114.
But a problem residing in the device is that once the natural frequency of the device is selected, it is not easy to alter the natural frequency. During a long service period, the natural frequency of the building and the natural frequency of the device may vary because of a deterioration of physical properties of the elements composing the structure or the device 110. In such cases, the device is not convenient in readjusting the natural frequency.
Another problem as to the device is that the device is effective for reducing a vibration in only one direction. Therefore, in order to reduce a three-dimensional vibration of a structure, it is obliged to install at least three such devices, each directed in a direction perpendicular to the others.